The present invention relates to the field of diagnostic imaging, and specifically to the field of positron coincidence imaging.
In nuclear imaging, a radiopharmaceutical such as .sup.99m Tc or .sup.201 T1 is introduced into the body of a patient. As the radiopharmaceutical decays, gamma rays are generated. These gamma rays are detected and used to construct a clinically useful image.
Positron emission tomography (PET) is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as .sup.18 F-Fluorodeoxyglucose (FDG) is introduced into the body of a patient. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays. The gamma rays are emitted in directions approximately 180.degree. apart, i.e. in opposite directions.
A pair of detectors registers the position and energy of the respective gamma rays, thereby providing information as to the position of the annihilation event and hence the positron source. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct a clinically useful image.
Various detector systems have been used in PET imaging. One class of PET systems can be termed non-rotating systems. The most common non-rotating systems have one or more rings of detector elements disposed in a circle about the patient. Other non-rotating systems include cylindrical shell detector systems and hexagonal multi-plate systems. In each of these systems, the detector surrounds or nearly completely surrounds the object to be scanned. Since coincidence events at substantially all transverse angles within a slice can be detected, system sensitivity is does not vary much between locations in a transverse slice.
Another class of PET systems can be termed rotating systems. Partial ring systems and dual or triple head gamma camera systems with coincidence detection capabilities fall into this category. One type of partial ring system includes two arcs of radiation sensitive detectors disposed on a generally circular rotating gantry. The arcs of radiation detectors are fixed with respect to each other so that their centers are generally diametrically opposed, with a slight angular offset. Rotating systems have partial transverse angle coverage such that it is necessary to rotate the detectors about the patient (or vice versa) in order to sample the transverse angles needed to reconstruct fully tomographic images. The sensitivity of these systems thus varies across the detector field of view. This variation in sensitivity is taken into account during processing of the coincidence data.
Nonetheless, the variations in sensitivity which are characteristic of rotating systems are undesirable for a number of reasons. Larger objects are not well covered because the transverse extent of the detector field of view is limited. Count statistics also vary significantly across the field of view, causing a variation in image quality. Also, with relatively fewer true counts available at various locations in the field of view, correction of random and scatter counts is complicated. The present invention addresses these problems, and others.